When semiconductor devices are formed in integrated circuits, the devices are coupled to each other through the use of back end of the line metal interconnect levels. The resistance and capacitance of these metal interconnect levels has a negative effect on device performance, especially at high frequency operation. Therefore, to minimize the influence of parasitic components (e.g., resistance, capacitance), layers of low electrical resistance and capacitance are used as electrodes to connect the overlying metal interconnect levels selectively to the individual active semiconductor elements.
These electrode layers are often comprised of monocrystalline silicon, polycrystalline silicon, and/or amorphous silicon and are doped as required to give them needed electrical conductivity. To form doped crystalline layers a semiconducting substrate (e.g. silicon) can be subjected to a series of chemical and thermal processing steps which modify the electrical properties of certain areas of the substrate. Typically, formation of the silicon layers is performed by first depositing undoped silicon. The undoped silicon is subsequently doped by means of ion implantation. Finally, a temperature step sometimes referred to as an anneal is used to heal implantation damages and electrically activate the dopant.
Depositing a single crystalline layer can be performed by first forming a seed layer and then epitaxially growing a layer with the same crystalline structure on top of the seed layer. Epitaxial silicon is usually grown using vapor-phase epitaxy (VPE), a modification of chemical vapor deposition (CVD). Molecular-beam and liquid-phase epitaxy (MBE and LPE) can also be used. The use of MBE and LPE is mainly done for compound semiconductors. During depositions performed by any of these methods, narrow faults in the underlying layer are often completely filled.
Doping places either specific amounts of dopant atoms into the crystal lattice structure of the substrate or a film deposited on the substrate. In general, the electrical characteristics (e.g., conductivity, resistivity) of a defined region of a semiconductor structure are a function of the concentration and depth of the dopants in that region. In order to obtain electrical devices having predictable and reliable electrical characteristics, a doping process is controlled to provide a desired concentration and depth for dopant atoms within the substrate. In the formation of an epitaxially grown electrode layer, implantation doping can cause device issues as the doping will not be able to extend to the bottom of the faults formed during deposition and as a result, the grown layers will not be uniformly doped.
In-situ doping, which introduces dopant atoms during the epitaxial growth process, provides advantages over implantation doping in terms of layer integrity. One common method of in-situ doping is by gas phase deposition (e.g., chemical vapor deposition (CVD)). With a CVD doping process, a deposition gas and a dopant gas are supplied to a process chamber of a CVD reactor. A substrate to be doped and the process chamber are maintained at a relatively high temperature. In the process chamber, the deposition gas and the dopant gas thermally decompose and deposit onto the substrate. The deposited film is thus a mixture of a deposition species and a dopant species. The dopant atoms move by filling empty crystal positions (i.e. vacancies) or alternately move through the spaces between the crystal sites (i.e. interstitial).
As an example of a prior art CVD doping process, polysilicon thin films are deposited along with a dopant on a silicon substrate. A suitable deposition gas for depositing polysilicon is silane. In general, the silane decomposes under the vacuum and the high temperature of the process chamber and deposits onto the substrate.
For in-situ n-type doping, common dopants include phosphorous and arsenic. These dopants respectively utilize dopant gases comprising phosphine (PH3) and arsine (AsH3) in a CVD doping process. However, as a result of utilization of any of these dopant gases for in-situ doping, the epitaxial deposition rate decreases considerably. Further, it is difficult to achieve dopant concentrations above 1e20/cm3 using in-situ doping. For in-situ p-type doping, a common dopant is boron which utilizes a diborane (B2H6) gas in a CVD doping process.